This invention relates to a bus-redundancy type apparatus for a transmitting system and, more particularly, to an apparatus having a twofold transmission line construction, so that one of the transmission lines can serve as a back-up to the other when the other is in trouble or undergoing inspection, or when it is necessary to back up the other transmission line for other reasons.
As examples of a bus-access control system for a multi-drop or bus type transmission line in a local area network with a plurality of stations connected to a transmission line, a token-passing system and a CSMA/CD (carrier sense multiple access/collision detection) system are well known in the art.
The token-passing system is one in which a token, as a permission to transmit, is given to particular stations to permit communication. In this sytem, therefore, the time necessary for moving the token constitutes an overhead, even if the transmission clock rate is increased to improve the transmitting speed. For this reason, there has been a limit on the improvement of the transmitting speed.
The CSMA/CD system, on the other hand, is one in which transmitting is allowed in the case of absence of a signal on the transmission line, that is, in the absence of communication, at which time no carrier is detected by carrier sensing. In this CSMA/CD system, the probability of collision of signals is increased with an increase in traffic. The system, therefore, is not suited for purposes requiring real time, e.g., voice communication and process control services. When there is a carrier, a station requiring to transmit must wait a predetermined period of time before effecting carrier sensing again. For this reason, the CSMA/CD system is not suited for a network, which is subject to congestion of transmitted signals and requires real time for communication.
As a means of providing a bus-access control system which can overcome the above drawbacks, an implicit token-passing system has been contemplated. In this system, are station is assigned for providing a synchronizing signal to the bus. Also, time slots are set for predetermined time intervals (which need not be constant) with respect to the synchronizing signal generated from the predetermined station, and specific time slots are allotted to the individual stations. Each station is allowed to transmit a signal during its allotted time slot. Each station can detect the time slots by carrier sensing, noted above with reference to the synchronizing signal, and it counts the detected time slots so that it can detect their order. When it detects its own allotted time slot, it transmits a signal together with information concerning the destination station.
In such a system, the number of different time slots is made equal to the number of stations present on the bus, and when the maximum count of time slots is reached, the synchronizing signal is generated once again to repeat the operation.
A feature of this system is that because each station has its own predetermined time slot, it can always transmit a signal for each cycle. Thus, it is possible to eliminate the inconvenience of waiting for permission to transmit after a collision of signals, as in the case of the CSMA/CD system. Also, since there is no need to shift the token, the overhead can be eliminated. Further, where communication is permitted among a plurality of stations using the above bus-access control system, such communication is possible during one cycle, as follows:
For the sake of simplicity, it is assumed that time slots TSn (n=1, 2, . . . ) are allotted to respective stations STn (n=1, 2, . . . ). The time slots may be freely allotted to the stations, so long as the same time slot is not allotted to two or more stations. In this system, it is possible to designate stations perticipating in the transmission and reception of signals, e.g., transmitting from station ST1 to station ST3 is designated by time slot TS1, transmitting from station ST2 to station ST1 is designated by time slot TS2, transmitting from station ST2 to station ST1 is designated by time slot TS3, and so forth.
In a data transmitting system such as a local area network, a redundant transmission line is usually provided to ensure reliability, as in the case where the system is to be applied to industrial control systems. The extent of redundancy provision varies, from system to system, e.g., from one in which all the constituent elements of a line are duplicated, to one in which only the transmission line is duplicated. However, due to considerations of economy and the trouble factor, the following system is usual; that is, a twofold transmission line construction consisting of two transmission lines, i.e., normally operating and stand-by transmission lines, and drivers and receivers coupled to the individual lines are also duplicated. When carrying out transmission, the same data is transmitted from a transmission controller to the twofold transmission line, through first and second switches. When engaged in reception, data on either one of the two transmission lines is selected through a third switch to be received in a reception controller. The transmission line interface of each station of the bus-access control system is constructed as aforementioned.
When applying each bus access control system noted above to a transmission system having a twofold transmission line structure, a CPU suitably switches the normally operating and stand-by transmission lines as the reception side transmission line. At this time, the timing of switching of the reception side transmission line is important. In the token-passing system noted above, the station which transmits the token governs the overall transmission. Therefore, a processor which controls the transmission can switch the transmission line according to a predetermined program. The timing of switching of the reception side transmission line can thus be readily determined.
In the case of the implicit token-passing system, on the other hand, each station continuously transmits such packets as frame header, agent packet, dummy packet, and data packet to the transmission line. Therefore, no problem occurs when the switching of the reception side transmission line is effected by a transmission control processor at a timing between packets, i.e., when there is no signal on the line. However, when the switching is performed in the presence of a packet, the received data is interrupted, resulting in an error.
Further, in the case of the CSMA/CD system, each station transmits at a desired timing, switching the reception side transmission line in the presence of a packet, and hence, the resultant generation of an error is liable to occur, as in the case of the implicit token-passing system.
When switching the transmission line in the above transmitting system, it is necessary to check, at the time of switching, whether the normally operating or stand-by transmission line is operative. For example, in a recent optical transmitting system using a star coupler featuring a noise-proof property, optical fiber cables are provided for transmission and reception for each station. To check for a defective part, therefore, it is necessary to transmit a check packet for each station, and confirm the presence of absence of the response.
Therefore, the time necessary for the check is increased with any increase in the number of stations, thus spoiling the high speed response of the bus-access system. Thus, a station for performing the check is limited to only one station.